The present invention relates to a method of improving, by spot welding, the quality of an austenitic stainless steel, the quality of which is degraded due to deterioration of its metal structure or cracking.
Occurrence of stress corrosion cracking in a structure made of austenitic stainless steel (hereunder referred to as a SUS structure) degrades the quality of the SUS structure. In order to improve the quality it is necessary to repair the SUS structure which has the cracking.
FIG. 32 is a diagram for explanation of the occurrence of stress corrosion cracking, and FIG. 33 is a sectional view of a butt-welded portion of a pipe and residual stress distribution in the section of the butt-welded portion.
As a conventional repairing technique when cracking has occurred in a SUS structure, in general, there is used a method of removing a cracked region by a device, such as a grinder, forming an edge-opening, and then welding the region for repairing it, or a method of melting the cracked region by continuous welding and then finishing the weld bead.
Further, an intergranular stress corrosion crack (hereunder, referred to as a SCC), which occurs at a welding heat affected zone of a nuclear reactor primary piping of SUS 304 in a BWR plant, occurs at the portion of high tensile stress 13 above 10 kgf/mm.sup.2, a chromium-depleted zone 14 formed along a grain boundary due to welding heat which causes sensitivity in the material, and at corrosive environment 15 of dissolved oxygen, etc.; and, these areas overlap, for example, to form a stress corrosion cracking occurrence region 16, as seen in FIG. 32.
In FIG. 33, a SUS piping 5 is welded to form a weld portion 6, with natural cooling, that is, without using forcible cooling, such as by causing water to flow in the piping.
In this case, as is apparent from a standard line or +10 kgf/mm.sup.2 line 10 in FIG. 33, such a high residual tensile stress reaching to several ten kgf/mm.sup.2 occurs in the inner surface of the piping.
Further, a chromium-depleted zone occurs in a welding heat affected zone 7, in this manner, when corrosive fluid flows along outer and inner side surfaces of the SUS piping 5 in which a high residual tensile stress and a chromium-depleted zone occur, and a danger that the SCC may take place in the welding heat affected zone 7 increases.
Therefore, for a machine or apparatus elements which come into contact with corrosive hot and high-pressure water containing therein dissolved oxygen, in particular, for a machine or apparatus elements for a BWR plant, high corrosion resistance is required for securing reliability and maintaining long life.
As one of the available conventional countermeasures for a SCC, a method of cladding corrosion resistive material containing delta ferrite on a surface which comes into contact with corrosive fluid and then applying a melting treatment on the cladded end portions is disclosed in JP B2 59-21711.
Substantially the same method as disclosed in the JP B2 59-21711 is also disclosed in U.S. Pat. No. 4,247,037.
The method extinguishes a chromium-depleted zone formed by heat affection in the cladding of corrosion resistant material and forms a structure containing delta ferrite and having excellent corrosion resistance.
Further, as another available countermeasure for a SCC, a method of applying solution treatment on one side surface of two side surfaces of an object while cooling the opposite side surface with respect to the one side surface is disclosed in JP A 2-258190.
The latter method forms a structure containing delta ferrite and having excellent corrosion resistance by applying the solution treatment on one side surface and reduces residual stress in the other side surface, whereby the corrosion resistance on both side surfaces is increased.
Further, as other countermeasures for a SCC, a method. of mounting a thin cylindrical sleeve on an inner surface of a piping for a BWR plant, and forming a structure containing delta ferrite and having excellent corrosion resistance by applying solution treatment thereon is disclosed in JP A 3-170093.
Substantially the same method as disclosed in JP A 3-170093 is disclosed in U.S. Pat. No. 5,227,124.
U.S. Pat. No. 4,562,332 discloses a technique in which electron beams are applied to cracks in a metal surface in a zigzag fashion to repair the cracks. However, it is not clearly disclosed in this US patent to form on the metal surface a structure containing delta ferrite and having excellent corrosion resistance.
The prior art does not take into consideration the following cases:
(1) a case where an allowable quantity of weld deformation due to repairing is small; PA1 (2) a case where a large depth of weld penetration is required when welding is carried out for repairing a crack because the depth of the crack is deep; PA1 (3) a case where an allowable quantity of undercut at a portion of the welding for repairing a crack is small for the reason that finishing work after the welding for repairing the crack is impossible, etc.; PA1 (4) a case where a large cooling speed is required for prevention of hot cracking during welding and prevention of stress corrosion cracking; PA1 (5) a case where it is necessary to change welding conditions according to the thickness of the welding portion because the thickness of the welding portion changes; and PA1 (6) a case where it is impossible to cause cooling water to flow onto a side surface opposite to a surface to be welded for repairing the surface to cool the opposite side surface.
In any of the above cases (1) to (6), repairing work for SUS structures is difficult.